The invention relates to a catalytic method of reducing chemical compounds in solution. The catalyst is a transition metal alcoholate complex. This catalytic complex may be produced and is regenerated by exposure to radiation of the appropriate wavelength.
It is well known that vanadium (II) is a strong reducing agent. However, the divalent ion is also the least stable and consequently the least important of the oxidation states of vanadium.
Transition metal ions, the best being the highly reactive vanadium (II), have been shown to reduce molecular nitrogen, acetylene, ethylene and carbon monoxide under certain conditions. The transition metal ions, being oxidized and consumed in the course of these reactions, are reducing agents and not true catalysts.
A research group heated by Schrauzer investigated reductions by vanadium (II). Zones, et al., "The Reduction of Molecular Nitrogen, Organic Substrates, and Protons by Vanadium (II)," J. Am. Chem. Soc. 98, 7289 (1976). In this article, the authors set forth the reduction of acetylene to ethylene and the reduction of ethylene to ethane by vanadium (II) hydroxide. They also indicate that hydrazine may be produced by the reaction of molecular nitrogen with alkaline suspensions of vanadium (II) hydroxide in magnesium hydroxide. In the absence of the magnesium hydroxide host lattice, only traces of hydrazine were formed by the reaction of nitrogen with vanadium (II) hydroxide. The addition of vanadium (III) to vanadium (II)/magnesium hydroxide systems prevented hydrazine formation and increased the evolution of hydrogen.
The group under the direction of Schrauzer also reduced acetylene, ethylene and 2-butyne respectively to ethylene or ethane, ethane and cis-2-butene using mixed hydroxide gels of vanadium (II)/magnesium hydroxide. Vanadium (II) was oxidized to vanadium (IV) during this reduction. Whereas vanadium (II) hydroxide had reduced nitrogen only in strongly alkaline media and in the presence of magnesium hydroxide to produce only traces of hydrazine and ammonia, in the form of a mixed hydroxide gel, at 0.degree.-25.degree. C., nitrogen was reduced virtually exclusively to hydrazine. At temperatures between 70.degree. and 90.degree. C., a secondary reduction of hydrazine to ammonia occured. Production of hydrazine was increased at elevated nitrogen pressures.
Schrauzer's group also reported on the effect of ultraviolet light in some of these reductions. They mention that the evolution of hydrogen, as well as the reduction of certain substrates by vanadium (II) in homogeneous acidic solutions, was stimulated by ultraviolet light. Irradiation of acidic solutions of vanadium (II) with ultraviolet light reduced acetylene, ethylene and, to a certain extent, nitrogen. The reactions of vanadium (II) hydroxide and of vanadium (II)/magnesium hydroxide with substrates were not markedly influenced by irradiation of the gels with ultraviolet light. Hydrogen evolution and reduction of acetylene, as well as ethylene in acidic solutions of vanadium (II) were stimulated by ultraviolet light. Traces of nitrogen were reduced to ammonia under similar conditions. Corresponding reactions occurred only very slowly, or not at all, in the dark or on irradiation with visible light. Whereas hydrogen evolution and ethylene and acetylene reduction were efficiently stimulated by ultraviolet light, the light-induced reduction of nitrogen produced only traces of ammonia under the same conditions.
The research by Schrauzer's group discussed above was directed to studies of the reducing properties of vanadium (II). Reactions of vanadium (II) as a reducing agent were studied. No attempt was reported to produce a catalytic reaction using divalent or trivalent vanadium as a catalyst, and not simply as a reducing agent.
A research group under the direction of Shilov studied the reduction of nitrogen by a vanadium (II)-catechol system. Nikonova, et al., "A Comparison of the Reduction of Dinitrogen by a Vanadium (II)-Catechol System with That by the Active Centre of Nitrogenase," J. Mol. Catal. 1, 367 (1976). Several molybdenum or vanadium containing compounds reduced molecular nitrogen to hydrazine and ammonia with the participation of solvent protons. A vanadium (II)-catechol complex reduced molecular nitrogen to ammonia in homogeneous aqueous and alcoholic media under mild conditions. The vanadium (II)-catechol complex is a strong reducing agent and reacted with the protons in the water molecules to form vanadium (III) and hydrogen. In the presence of nitrogen at a pH of 8.5-14, both in aqueous and alcohol solutions, a competitive reduction of molecular nitrogen to ammonia occurred. This reduction of nitrogen was inhibited by carbon monoxide and acetylene. Hydrazine was readily reduced to ammonia by vanadium (II)-catechol complexes.
The research by this group under the direction of Shilov was also directed to the study of vanadium (II) as a reducing agent. In this research, as in that of Schrauzer's group, the well-established reducing properties of divalent vanadium were further investigated.
A number of prior workers have attempted to reproduce the process of nitrogen "fixation," the naturally-occurring reduction of molecular nitrogen to ammonia by certain types of plants through a process which is believed to be enzymatic. For example, another group under the direction of Shilov investigated the reduction of nitrogen in protonic media in the presence of several metal compounds. Shilov, et al., "New Nitrogenase Model for Reduction of Molecular Nitrogen in Protonic Media," Nature 231, 460 (June 18, 1971). In the laboratory, titanium (III), chromium (II) and vanadium (II) were shown to reduce molecular nitrogen to both hydrazine and ammonia in aqueous and alcohol solutions. The reduction by titanium (III) and chromium (II) proceeded only in the presence of molybdenum compounds. The publication states, on the other hand, that vanadium (II) reduced nitrogen in the absence of molybdenum compounds, and formed the most active systems for reducing molecular nitrogen. The reduction of nitrogen proceeded only at a pH greater than 7. Alkaline solutions containing titanium (III), chromium (II) and vanadium (II) in the absence of molybdenum compounds also reduced acetylene to ethylene.
Although Shilov demonstrated that several metals, including vanadium (II), were capable of reducing molecular nitrogen, acetylene or ethylene, no true catalytic acticity was established. Again in these studies the well-known properties of divalent vanadium as a reducing agent were investigated.
Koryakin, et al. studied the photo-reduction of vanadium (III) to the divalent state in water-alcohol solutions. "Photocatalytic Liberation of Hydrogen from Water-Alcohol Solutions of Vanadium Trichloride," Dokl. Akad. Nauk SSSR 229, 128 (1976). This process was further investigated by Applicants. Doi, Y and M. Tsutsui, "Fluorescence and Photochemistry of the Charge-Transfer Band in Alcoholic Vanadium Trichloride Solution," J. Amer. Chem. Soc. 100, 3243 (1978). Photo-reduction was observed to be accompanied by the simultaneous catalytic formation of hydrogen and the oxidition of the alcohol to an aldehyde. Vanadium (III) alcoholate complexes in the parent alcohol solution exhibited luminescence of relatively high quantum yield upon excitation at the charge-transfer band with a competitive photo-reduction of vanadium to the divalent state.
Methods of catalytically reducing molecular nitrogen to ammonia have long been sought. Although the strong reducing activity of divalent vanadium has long been known, and studies indicated its capability to reduce molecular nitrogen, the divalent vanadium was stoichiometrically consumed in the reduction. Thus, any reduction employing divalent vanadium as the reducing agent required the continual replenishment of the consumed reactant, vanadium (II).